The present invention generally relates to digital measurements of voltage phasors for protective relaying purposes in situations when voltage signals are brought to a protective relay via Capacitive Voltage Transformers (CVTs). More particularly, the present invention improves the dynamic accuracy of phasor measurements so that the distance and directional protection elements of microprocessor-based relays are faster and more accurate.
Microprocessor-based protection devices for power systems operate by sampling the input currents and/or voltages at regular time intervals, measuring digitally selected features of those signalsxe2x80x94primarily magnitudes and phase anglesxe2x80x94and comparing the signal features one against another or against thresholds. For proper functioning, the voltage and current phasors must be measured quickly and accurately.
In High Voltage (HV) and Extra High Voltage (EHV) power systems, CVTs are often used to reduce the voltages from the range of hundreds of thousands of volts (primary voltage level) to tens of volts (secondary voltage level) before supplying the voltage signals to measuring and protective devices. CVTs are typically cheaper than magnetic voltage transformers, but cause certain problems for protective relays as they add specific transient components to the original high voltage signals when transforming the voltages to the secondary level.
U.S. Pat. No. 3,870,926 discloses a typical CVT consisting of a capacitive voltage divider, tuning reactor, step-down transformer and ferroresonance suppression circuit. During faults on power system transmission lines, when the primary voltage collapses and the energy stored in the stack capacitors and the tuning reactor of a CVT is to be dissipated, the CVT generates severe transients affecting performance of protective relays.
CVT-generated transients tend to have relatively significant magnitudes and long durations. This becomes particularly important for relays protecting transmission lines when the Source Impedance Ratio (SIRxe2x80x94the ratio between the system equivalent impedance and the relay reach impedance) is large. Under large SIRs the primary voltage during line faults is very low. This signal is crucial for protective relays to operate properly, but it is significantly distorted by components generated not by the power system itself, but by the CVTs. Generally, the CVT generated transients are referred to as the d.c. (direct current) component, although, as a matter of fact, there may be multiple components generated by a CVT and some of them may be oscillatory (a.c. (alternating current) components).
CVT transients affect all the voltage related functions of a protective relay. It is particularly important, however, for distance and directional functions.
When the well-known Fourier algorithm is applied to voltage phasor measurements, which is typical in microprocessor-based relays, the magnitude of the voltage may be significantly underestimated due to the CVT transients. This causes distance elements of the relay to malfunction.
Likewise, the phase angle of the voltage phasor is not measured precisely due to the CVT transients. This causes the directional elements of the relay to malfunction.
Electromechanical relays cope with unfavorable CVT transients thanks to the natural mechanical inertia at the expense of slower operation.
Known methods for dealing with CVT transients in microprocessor-based protective relays is to introduce certain, either fixed or adaptable, delay into a relay, or to reduce the reach of distance elements either permanently or in an adaptive manner.
U.S. Pat. No. 4,763,068 discloses an apparatus for measuring the alternating component of the voltage signal supplied via a CVT. The approach is based on an analog circuit, and as such is not a cost efficient mean of dealing with CVT transients in microprocessor-based protective relays. In addition, the method assumes a d.c. nature of the CVT transient when reconstructing the disturbing component and subtracting it from the input voltage in order to obtain the a.c. component alone. As mentioned above, some CVTs produce oscillatory transients, and consequently, the method disclosed in the ""068 patent is not accurate for such CVTs.
U.S. Pat. No. 5,729,477 discloses a method for eliminating a disturbing component from CVT supplied voltage signals. The method is intended to be implemented on a microprocessor-based device and relies on calculating the parameters of the d.c. component present in the input signal and subtracting the reconstructed d.c. component from the input signal. Since the method requires digital measurements at two time instances only, it is clear that the method can deal only with one component, this component must be an exponentially decaying d.c. component and its time constant must be known. Because of the above limitations, the method described in the ""477 patent does not address the aforementioned problems sufficiently.
U.S. Pat. No. 4,196,388 discloses an apparatus for removing disturbing components from the CVT supplied voltage signal by means of switchable analog filters. The apparatus uses two different analog filter modes: one with wide frequency response (and consequently, fast time response), and one with narrow bandwidth (and consequently, slow time response). The apparatus includes a mechanism for automatic control of the bandwidth. By narrowing the bandwidth of the filter during power system faults, the apparatus introduces certain delay to the voltage signal. This affects performance of protective relays by slowing them down. In addition, the apparatus, as an analog device, can not be used directly by microprocessor-based relays on a cost efficient basis.
U.S. Pat. No. 4,437,134 discloses an apparatus for fast discharge of the energy trapped in the stack capacitors of a CVT. The apparatus uses a special circuit comprising of semiconductor devices and a separate detector switching the first circuit on when needed. The apparatus, as an analog device, is rather an enhancement of CVT design, and cannot be used directly by microprocessor-based relays on a cost efficient basis to cope with problems caused by regular CVT designs.
In view of the above, it would highly desirable to provide a numerical algorithm for pre-filtering a CVT supplied voltage signal which would provide good filtering regardless of the CVT type including values of stack capacitors, type of the ferroresonance suppression circuit, type and value of the burden. It would further be described to provide a filter which introduces minimal time delay. It would also be desirable for a filter to provide optimal performance both magnitude-wise and phase-wise of the commonly used Fourier algorithm when the latter is applied to the pre-filtered voltage signal.
To achieve these goals, and provide other advantages, embodiments of the present invention provide a linear Finite Time Response (FIR) digital filter to pre-filter CVT supplied voltages prior to application of the Fourier algorithm and other functions of a microprocessor-based protective relay.
The filter can be designed as a cascade of two FIR filters. A first stage of the filter suppresses decaying d.c. components and the oscillatory decaying components of the frequency lower than the power system frequency (50 or 60Hz). A second stage of the filter provides a dynamic memory by using certain number of historical samples of the voltage signal, averaging the results, and using the average to effectively compose the output signal of the filter.
The filter does not need to be tuned to a particular CVT; thus the CVT characteristic is not necessary to apply the filter, and consequently, the filter is universal.
The filter introduces minimal time delay, and as such, it does not slow down the operation of protective relays unnecessarily.
As a result of pre-filtering the magnitude of the voltage phasor as measured by the Fourier algorithm is underestimated only minimally, and the phase angle of the voltage phasor is measured much more accurately compared to the Fourier algorithm used on raw samples of the CVT supplied input voltage.
A microprocessor-based relay implementing the presented invention provides improved performance of distance and directional protection elements.